The field of the invention is that of zeolite adsorbents for the separation of xylenes, in particular with a view to the industrial production of para-xylene.
High-purity para-xylene is reclaimed industrially, inter alia, in order to be converted into terephthalic acid intended for the manufacture of PET.
The prior art has recognized that adsorbents consisting of zeolites X or Y exchanged using ions such as barium, potassium or strontium, alone or as a mixture, are efficient at selectively adsorbing para-xylene in a mixture containing at least one other aromatic C8 isomer. U.S. Pat. Nos. 3,558,730, 3,558,732, 3,626,020 and 3,663,638 disclose adsorbents comprising aluminosilicates exchanged with barium and potassium, which efficiently separate para-xylene from a mixture of aromatic C8 isomers. These adsorbents are used as adsorption agents in liquid-phase processes, preferably of the simulated counter-current type similar to those described in U.S Pat. No. 2,985,589, which are applied, inter alia, to the aromatic C8 fractions derived, for example, from processes for the dialkylation of benzene, in gas-phase processes.
The performance of the industrial process for the separation of para-xylene depends largely on the adsorbent, on its adsorbing capacity and on the selectivity which it shows-for para-xylene in a medium consisting of C8 aromatics, typically para-xylene itself (PX), meta-xylene (MX), ortho-xylene (OX) and ethylbenzene (EB), as well as on the ability of desorbents, such as toluene and para-diethylbenzene, to desorb the adsorbed para-xylene therefrom.
The selectivity Sel(B/A) of the adsorbent for a compound (B) relative to a compound (A) is defined as the ratio of the concentrations of the compounds in the adsorbed phase divided by the ratio of concentrations of the compounds in the non-adsorbed phase at equilibrium.
The selectivity equation is as follows:       Sel    ⁡          (              B        /        A            )        =                    (        B        )            ⁢              z        /                  (          A          )                    ⁢      z                      (        B        )            ⁢              s        /                  (          A          )                    ⁢      s      
where (B)z and (B)s represent the concentrations of B in the zeolite and in the solution, respectively, where (A)z and (A)s represent the concentrations of A in the zeolite and in the solution. The method for evaluating these magnitudes is outlined later.
The zeolites encountered in the prior art for the separation of xylenes belong to the faujasite structural type, first described in U.S Pat. No. 2,882,244 and U.S Pat. No. 3,130,007, which are crystalline silicoaluminates having cages of fully defined size connected three-dimensionally. The faujasites correspond to the general formula:
(1xc2x11) M2/nO ; Al2O3; W SiO2; Y H2O
in which
M represents at least one alkali metal cation or alkaline-earth metal cation of valency n,
Y is less than or equal to 8 depending on the nature of M and the degree of hydration of the crystal,
W is the factor which makes it possible to distinguish between silica-rich faujasites (faujasites Y) and alumina-rich faujasites (faujasites X).
Faujasites X should be placed in the range Wxe2x89xa63, and faujasites Y in the range W greater than 3, which is interpreted, rather, according to the Si/Al ratio on either side of 1.5. For the purposes of the present invention, an additional convenient distinction is introduced with faujasites with a low silica content (which will be referred to by the abbreviation LSX used by those skilled in the art, meaning Low Silica X), for W less than 2.3 (Si/Al less than 1.15).
The prior art has recognized, as adsorbent for the separation of xylenes, only those faujasites exchanged with barium with Si/Al atomic ratios of between 1.2 and 2.6. In this matter, a person skilled in the art does not appear to have available any simple and sufficiently reliable criterion for predicting their behaviour. Thus, the search for increasingly improved adsorbents proceeds by more or less random research, such as, for example, the result disclosed and claimed by U.S Pat. No. 3,878,127 with an actual barium exchange of zeolite X pretreated with a sodium hydroxide solution. The Applicant proposes adsorbents which offer para-xylene/meta-xylene or para-xylene/ortho-xylene selectivities of at least 2.0 and, advantageously, of least 2.5, measured according to the test described in the examples. The invention achieves this result, with a number of advantages which will become apparent in the description.
The subject of the present invention is agglomerated zeolite adsorbents comprising at least 70% and preferably at least 80% of faujasite with an Si/Al atomic ratio such that 1xe2x89xa6Si/Alxe2x89xa61.15, in which the exchangeable sites are at least 70% occupied by barium ions and optionally up to 30% by potassium (any remainder generally being made up of alkali metal ions or alkaline-earth metal ions other than barium and potassium) and agglomerated with a binder, the preferred faujasites being those in which the overall level of exchange for barium alone or for barium+potassium is greater than or equal to 90%.
One process for the preparation of the agglomerated zeolite adsorbents according to the invention consists, firstly, in agglomerating zeolite powder with an Si/Al ratio such that 1xe2x89xa6Si/Alxe2x89xa61.15 with a binder, preferably a zeolitizable binder.
The term agglomeration is understood to refer to the production of solid particles from a mixture of zeolite(s) and binder(s) using any technique known to those skilled in the art, such as extrusion, granulation, compacting or spraying. The practical content of binder in the agglomerate generally does not exceed 30%, and preferably 20%, of the total mass of the adsorbent. The efficiency of these adsorbents is improved substantially by selecting a clay of the kaolin family as agglomeration binder, in practice kaolinite or halloysite, and by subjecting the granules to zeolitization.
Zeolitization of the binder is performed by immersing the agglomerate in an alkaline liquor, sodium hydroxide or a mixture of sodium hydroxide and potassium hydroxide whose concentration is preferably at least 0.5 M, after the grains have been calcined, the first result of this first calcination being to harden the grain, but also to activate the clay by converting it into meta-kaolin. The zeolitization is preferably carried out under hot conditions since working at the higher temperature improves the kinetics of the process and reduces the immersion times. Zeolitizations of at least 50% of the binder, that is to say that the resulting adsorbent generally consists of at least 85% and preferably at least 90% zeolite of active faujasite type and of not more than 15%, preferably not more than 10% of material which is inactive for the adsorption, are thus readily obtained.
The barium exchange is performed in an entirely conventional manner, preferably by successive exchanges so as to reach a minimum target exchange level of at least 70% and preferably at least 90%.
The potassium exchange can be performed before or after the barium exchange, but it is also possible to agglomerate faujasite LSX powder already containing potassium ions.
The activation is the final step in the production of the adsorbents of the invention. Its aim is to fix the water content, more simply the loss on ignition of the adsorbent, within optimal limits. The most practical way of proceeding is by thermal activation, which is preferably carried out between 180 and 250xc2x0 C.
The invention also consists of an improvement to the process for recovering para-xylene from aromatic C8 isomer fractions, which consists in using, as adsorption agent, a zeolite adsorbent based on faujasite with an Si/Al ratio such that 1xe2x89xa6Si/Alxe2x89xa61.15, in which the exchangeable sites are at least 70% occupied by barium ions (the remainder being made up of alkali metal ions or alkaline-earth metal ions other than barium) agglomerated with a binder, preferably a zeolitizable binder. The agglomerated zeolite adsorbents according to the invention are suitable when they are used in processes in the liquid phase or in the gas phase.
The desired product can thus be separated out by preparative adsorption liquid chromatography (batchwise), advantageously in a simulated fluid bed, i.e. one with a simulated counter-current or a simulated co-current, and more particularly with a simulated counter-current.
The operating conditions of an industrial unit for adsorption of simulated counter-current type are generally as follows:
(for example 1.4 to 1.8 for an adsorption unit alone (stand-alone) and 1.1 to 1.4 for an adsorption unit combined with a crystallization unit)
Reference may be made to U.S. Pat. Nos. 2,985,589, 5,284,992 and 5,629,467.
The operating conditions for a simulated co-current adsorption industrial unit are generally the same as those operating in simulated counter-current, except for the level of recycling, which is generally between 0.8 and 7. Reference may be made to U.S Pat. Nos. 4,402,832 and 4,498,991.
The desorption solvent can be a desorbent whose boiling point is less than that of the feedstock such as toluene, but also a desorbent whose boiling point is greater than that of the feedstock, such as para-diethylbenzene (PDEB).
Faujasites with an Si/Al ratio more or less equal to 1, which are prepared according to the production method described in European patent EP 486,384 or U.S Pat. No. 5,173,462, are preferred here. The selectivity of the adsorbents according to the invention for the adsorption of the p-xylene contained in C8 aromatic fractions is optimal when their loss on ignition, measured at 900xc2x0 C., is generally between 4.0 and 7.7%, and preferably between 5.2 and 7.7%. Water and a small amount of carbon dioxide form part of the loss on ignition.
The non-limiting examples which follow will allow the invention to be understood more clearly.